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The Rh protein shot is routinely given to pregnant women who areRh-negative. These women lack the Rh protein on the surface of their red bloodcells.B The injection prevents Rh disease, which can cause the fetus to bestillborn or lead to fatal illnesses in the newborn.

It has been suggested that children born to mothers who are given injectionsof Rh protein during pregnancy may be at increased risk for autism becausethimerosal was used in the Rh protein injection until 2001.

"We hypothesized that if thimerosal were associated with the developmentof autism, we would find a higher proportion of Rh negative mothers of childrenwith autism born before 2002," University of Missouri researchers Judith H.Miles, MD, PhD, and T. Nicole Takahashi write in the May issue of theAmerican Journal of Medical Genetics.

'Time to Move On'

To test the theory, the two researchers determined the Rh status of 214mothers who had children with an autism spectrum disorder and reviewed medicalrecords to determine if the Rh-negative women had received the Rh proteinvaccine late in pregnancy.

Rh-negative status was found to be no higher in the mothers of the autisticchildren than in the general population, nor was there more exposure tothimerosal-containing Rh vaccine in autistic children prior to birth.

Though she knows the findings won't change the minds of those convinced thatthimerosal exposure causes autism, Miles tells WebMD that there is still nocredible scientific proof of a link.

In 2004, an expert panel convened by the Institute of Medicine (IOM) toreview the data on vaccines and autism came to the same conclusion.

"It is time to move on and focus our research dollars and efforts onavenues that will be more productive," Miles says.

Thimerosal Still in Vaccines

While the Rh vaccine and most vaccines given to children in the U.S. nolonger contain thimerosal, the inexpensive preservative is still widely used tokeep vaccines affordable in other parts of the world.

The researchers say that their study and others like it should reassurethose who still question the safety of vaccines containing thimerosal.

"We hope this report of no association between autism, Rh negativity andthimerosal exposure during pregnancy will offset some of the decreasedcompliance with immunization recommendations which is known to increasemorbidity and mortality from childhood infectious diseases," theywrite.